2.1. Ciliary Neurotrophic Factor
Ciliary neurotrophic factor (CNTF) is a protein that is specifically required for the survival of embryonic chick ciliary ganglion neurons in vitro (Manthorpe et al., 1980, J. Neurochem. 34:69-75). The ciliary ganglion is anatomically located within the orbital cavity, lying between the lateral rectus and the sheath of the optic nerve; it receives parasympathetic nerve fibers from the oculomotor nerve which innervate the ciliary muscle and sphincter pupillae.
Ciliary ganglion neurons have been found to be among the neuronal populations which exhibit defined periods of cell death. In the chick ciliary ganglion, half of the neurons present at embryonic day 8 (E8) have been observed to die before E14 (Landmesser and Pilar, 1974, J. Physiol. 241:737-749). During this same time period, ciliary ganglion neurons are forming connections with their target tissues, namely, the ciliary body and the choriod coat of the eye. Landmesser and Pilar (1974, J. Physiol. 241:751-736) observed that removal of an eye prior to the period of cell death results in the complete loss of ciliary ganglion neurons in the ipsilateral ganglion. Conversely, Narayanan and Narayanan (1978, J. Embryol. Ex. Morphol. 44:53-70) observed that, by implanting an additional eye primordium and thereby increasing the amount of available target tissue, ciliary ganglion neuronal cell death may be decreased. These results are consistent with the existence of a target derived neurotrophic factor which acts upon ciliary ganglion neurons.
In culture, ciliary ganglion (CG) neurons have been found to require a factor or factors for survival. Ciliary neurotrophic factor(s) (CNTF) activity has been identified in chick muscle cell conditioned media (Helfand et al., 1976, Dev. Biol. 50-541-547; Helfand et al., 1978, Exp. Cell Res. 113-39-45; Bennett and Nurcome, 1979, Brain Res. 173:543-548; Nishi and Berg, 1979, Nature 277-232-234; Varon et al., 1979, Brain Res. 173:29-45), in muscle extracts (McLennan and Hendry, 1978, Neurosci. Lett. 10:269-273); in chick embryo extract (Varon et al., 1979, Brain Res. 173:29-45; Tuttle et al., 1980, Brain Res. 183:161-180), and in medium conditioned by heart cells (for discussion, see also Adler et al., 1979, Science 204:1434-1436 and Barbin et al., 1984, J. Neurochem. 43:1468-1478).
Adler et al. (1979, Science 204:1434-1436) used an assay system based on microwell cultures of CG neurons to demonstrate that a very rich source of CNTF was found in the intraocular target tissues the CG neurons innervate. Out of 8000 trophic units (TU) present in a twelve-day embryo, 2500 TU were found present in eye tissue; activity appeared to be localized in a fraction containing the ciliary body and choroid coat.
Subsequently, Barbin et al. (1984, J. Neurochem. 43:1468-1478) reported a procedure for enriching CNTF from chick embryo eye tissue. CNTF activity was also found to be associated with non-CG tissues, including rat sciatic nerve (Williams et al., 1984, Int. J. Develop. Neurosci 218:460-470). Manthorpe et al. (1986, Brain Res. 367:282-286) reported partial purification of mammalian CNTF activity from extracts of adult rat sciatic nerve using a fractionation procedure similar to that employed for isolating CNTF activity from chick eye. In addition, Watters and Hendry (1987, J. Neurochem. 49:705-713) described a method for enriching CNTF activity approximately 20,000-fold from bovine cardiac tissue under non-denaturing conditions using heparin-affinity chromatography. CNTF activity has also been identified in damaged brain tissue (Manthorpe et al., 1983, Brain Res. 267:47-56; Nieto-Sampedro et al., 1983, J. Neurosci. 3:2219-2229).
Carnow et al. (1985, J. Neurosci. 5:1965-1971) and Rudge et al., (1987, Develop. Brain Res. 32:103-110) describe methods for identifying CNTF-like activity from Western blots of tissue extracts and then identifying protein bands containing CNTF activity by inoculating the nitrocellulose strips in a culture dish with CG neurons and identifying areas of cell survival using vital dyes. Using this method, Carnow et al. (1985, J. Neurosci. 5:1965-1971) observed that adult rat sciatic nerve and brain-derived CNTF activities appear to exhibit a different size (24 kD) than chick CNTF (20.4 kD).
Recently, CNTF has been cloned and synthesized in bacterial expression systems, as described in U.S. patent application Ser. No. 07/570,651, entitled "Ciliary Neurotrophic Factor," filed Aug. 20, 1990 by Sendtner et al. incorporated by reference in its entirety herein. Using recombinant probes, CNTF-mRNA in tissues of adult rat appeared to be about 1.2 kb in size. Rat brain CNTF was cloned and found to be encoded by a mRNA having a short 5' untranslated region of 77 bp and an open reading frame of 600 bp, predicting a protein of about 200 amino acids (Stockli et al., 1989, Nature 342:920-923). Human CNTF was also cloned and sequenced (U.S. patent application Ser. No. 07/570,651, entitled "Ciliary Neurotrophic Factor," filed Aug. 20, 1990 by Sendtner et al.); its coding sequences were substantially conserved relative to rat sequences, whereas noncoding sequences were less conserved.